Hitherto, as the element member of semiconductor device, photosensitive device for use in electrophotography, image input line sensor, image pickup device, or other optical devices, there have been proposed a number of amorphous semiconductor films, for instance, an amorphous deposited film composed of an amorphous silicon material compensated with hydrogen atom or/and halogen atom such as fluorine atom or chlorine atom [hereinafter referred to as "A-Si(H,X)"]. Some of such films have been put to practical use.
Along with those amorphous semiconductor films, there have been proposed various method for their preparation using plasma chemical vapor deposition technique wherein a raw material is decomposed by subjecting it to the action of an energy of direct current, high frequency or microwave to thereby form a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum. And there have been also proposed various apparatuses for practicing such methods.
Now, in recent years, the public attention has been focused on plasma chemical vapor deposition by way of MW-PCVD process.
One representative apparatus for practicing such plasma chemical vapor deposition, for example, for preparing a photoelectrographic photosensitive member, is such as disclosed in European Patent Publication No. 154,160 A1 that has a structure as shown by a schematic explanatory view of FIG. 3(A) and its X--X line cross-sectioned explanatory view of FIG. 3(B).
In FIGS. 3(A) and 3(B), there are shown a substantially enclosed reaction chamber 101, a microwave transmissive window 102 which is made of dielectric material such as alumina ceramics or quartz, a waveguide 103 which transmits a microwave from a microwave power source (not shown), an exhaust pipe 104 being connected through an exhaust valve (not shown) to an exhaust apparatus (not shown), and a plurality of cylindrical substrates 105 onto which a deposited film is to be formed and each of which being supported on a rotatable substrate holder having an electric heater 107 therein and being mechanically connected to a motor (not shown).
Numeral 106 stands for discharge space into which raw material gases are supplied from gas feed means (not shown) being mounted in the position behind the substrates 105 and which are connected to gas reservoirs (not shown).
In this apparatus, the reaction chamber has a structure of cavity resonator to resonate a frequency oscillated from the microwave power source (not shown) since the discharge is conducted upon self-induced discharge without using a trigger.
The film forming process using the apparatus shown in FIGS. 3(A) and 3(B) is carried out, for instance, in the following way.
That is, the air in the reaction chamber 101 is evacuated by opening the exhaust valve of the exhaust pipe 104 to bring about the inside to a desired vacuum. Then, the heater 107 is actuated to uniformly heat the substrate 105 to a desired temperature and it is kept at this temperature. Concurrently, the motor (not shown) is started to rotate the substrates 105 and they are kept rotating at a desired constant rotation speed.
Successively, in the case of forming an amorphous silicon film for example, silane gas (SiH.sub.4) and H.sub.2 gas are supplied through the gas feed means into the reaction chamber 101 at respective desired flow rates. After the flow rates of the raw material gases will have become stable, a microwave of more than 500 MHz or preferably of 2.45 GHz from the microwave power source is supplied through the waveguide 103 and the microwave transmissive window 102 in the reaction chamber 101, wherein the raw material gases are excited with a microwave energy as supplied to generate plasmas containing neutral radical particles, ion particles, electrons, etc. The thus resulted plasmas become mutually reacted to thereby form a deposited film on the surface of each of the rotating substrates 105.
Explaning the film forming process in this case by reference to FIG. 3(B), part of the surface of the substrate 105 to become situated in the front region (a) of the discharging space 106 will be exposed to an atmosphere containing uniformly distributed plasmas and because of this, a film will be uniformly deposited thereon (this film will be hereinafter called "front film"). On the other hand, other parts of the surface of the substrate 105 to become situated in the side regions (b) of the discharging space 106 will be exposed to an atmospheres containing unevenly distributed plasmas, so that films to be deposited on such other parts of the surface of the substrate will become uneven accordingly (these films will be hereinafter called "side part films"). The remaining part of the surface of the substrate 105 to become situated in the non-discharging back region (c) will not be exposed to plasma, so that said part will be maintained without being deposited with any film in said region.
In this respect, the resulting films will often become such that have defects in uniformity and also in homogeneity and that are not satisfactory in characteristics required for the light receiving layer of a photosensitive device, for example.
Therefore, there still remains an unsolved problem for the above-mentioned known film forming process that it is difficult to stably obtain a desired film suited for use as a constituent layer in semiconductor devices, photosensitive devices for use in electrophotography, image input line sensors, image pickup devices, photoelectromotive force devices or the like.
In particular, there is a problem for the above-mentioned known process in the case of mass-producing a large size electrophotographic photosensitive member having a light receiving layer of large area that it is extremely difficult to stably obtain a desired large size electrophotographic photosensitive member of uniform quality with a high yield.